Achromatic depth-of-field correction for off-axis optical system

ABSTRACT

Unmodulated, collimated white light is angularly directed to a target reflecting surface which is deformable to represent a subject, and the subject-modulated light reflected from the target surface is directed to a viewing screen by a schlieren optical system including a projection lens and a stop at the focal point of the lens. A prism is located adjacent the real subject-bearing target surface for rendering the apparent target surface parallel, and the light reflected therefrom orthogonal, to the principal plane of the lens and the screen; and for so reducing any color dispersion of the reflected light as to obtain adequate resolution of the image of the subject projected onto the screen.

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United States Patent 1 3,704,936 Gorkiewicz et al. SUBSTITUTE FORlVllSSlNG OR 1 Dec. 5, 1972 [s41 ACHROMATIC DEPTH-OF-FIELD 3,200,2508/1965 Bouwers ..350/l82 x CORRECTION FOR OFF-AXIS 2,647,437 8/1953Bentley et al ..353/67 x OPTICAL SYSTEM Primary Examiner-Ronald L.Wibert [72] Inventors: Walter Joseph Gorkiewicz, New

York, N.Y., John A. van Raalte, jsmmm g wi'. M cGraw Princeton, NJttorneyugene itacre [73] Assignee: RCA Corporation [57] ABSTRACT Filed!March 9, 1970 Unmodulated, collimated white light is angularly [2!] APPLN0Z 17,671 directed to a target reflecting surface which is deformableto represent a sub ect, and the sub ectmodulated light reflected fromthe target surface is [52] US. Cl. ..350/l6l, 350/182, 350/204, directedto a viewing Screen by a Schlieren optical 353/69 353/81 systemincluding a projection lens and a stop at the [5] 1 'f CL "Gozf 1/28Gozb G03b 21/00 focal point of the lens. A prism is located adjacent the[58] Field of Search ..35 0/l6l, 168, 202, I82, 28 al subject-bearingtarget surface for rendering the 350/204 353/69 33 apparent targetsurface parallel, and the light reflected therefrom orthogonal, to theprincipal plane of the [56] References cued lens and the screen; and forso reducing any color UNITED STATES PATENTS dispersion of the reflectedlight as to obtain adequate resolution of the image of the subject proected onto 2,510,846 6/1950 Wikkenhauser ..350/l6l X the Scream3,527,522 9/1970 Baumgartner ..350/l6l 2,338,488 H1944 Brown 9 Claims, 4Drawing Figures ACHROMATIC DEPTH-OF-FIELD CORRECTION FOR OFF-AXISOPTICAL SYSTEM At present, bright television images are best produced byprojecting onto a viewing screen light modulated by a light valve torepresent a subject. One such light valve is a multi-facetedlightreflecting film which is used in a schlieren type of opticalprojection system. In such a system either one of two generalarrangements has been employed. In one arrangement a single lens hasbeen used to perform (l) the function of collimating the light from asource which is directed onto the subject-bearing film which is deformedto a small degree and in different amounts to reflectively deviate theimpinging light rays so as to represent the subject and (2) the functionof projecting the reflected light toward the viewing screen. In such anarrangement a stop is placed at the focal point of the lens so as toprevent any undeviated reflected light rays received from thesubject-bearing film from reaching the screen. While. in thisarrangement, the principal reflected light rays are substantiallyorthogonal to the principal plane of the lens and to the screen and thedefinition of the projected image is satisfactory, the contrast of theimage tends to be undesirably reduced by unwanted light which isreflected toward the screen by the single lens serving both ascollimator and as projector.

In the other previously used general arrangement separate collimator andprojector lenses have been employed. In this type of arrangement thecollimated and projected light beams necessarily are at angles to thereflecting surface which is deformed to represent the subject. Whilesuch an arrangement can produce a projected image having relatively highcontrast, it is one in which the image field undesirably deviates fromorthogonality to the axis of the imaging light beam which produces adepth-of-error. Such an error is manifested by an inability to projectthe image onto the screen with all parts of it in good focus. Forexample, such systems may be adjusted so that a central band of theprojected image is in good focus on the viewing screen but the top andbottom portions of the image will be out of focus. Hence, thesimultaneous requirements of satisfactory imaging of the light source onthe schlieren stop and of producing good quality (i.e., low aberrations)of the image projected onto the viewing screen can only be satisfied forrelatively small optical apertures and, consequently, the projectedimage will have an unsatisfactorily low brightness.

The use of relatively large optical apertures in a twolens system causesa deviation from orthogonality of the reflected light rays on the screenwhich produces a keystone distortion of the image and a degradingvariation of the resolution of the projected image. Heretofore, variousmeasures have been adopted to mitigate to some degree one or both ofsuch defects. One expedient was to predistort the dimensions of theimage to be projected so as to compensate for the expected keystoning.Such an approach requires circuit modifications which not only add tothe cost of the equipment but also produce other undesired imagedegradation. Another technique which has been employed is to effect thecompensation by means of a fiber-optic combination prism. Such apparatusnecessarily is quite complicated and cumbersome in addition to being-,costly. .A third suggested compensationcleviceis-that disclosed inU.S. Pat. 3,249,004 issued May 3, 1966 to O. A. Ullrich which comprisestwo glass blocks located in the projected light path and serving toproduce a rough depth-of-field compensation. Not only is thecompensation afforded by such means too crude for the relatively highdegree of resolution needed for the projection of good qualitytelevision images but also unwanted lines, produced by the junction ofone block with another, may appear in the image projected onto theviewing screen.

It is an object of this invention, therefore, to provide simple andinexpensive means for effecting depth-offield correction in an off-axisoptical projection system which has none of the deficiencies of theprior art compensators of the character described and which issubstantially achromatic.

The present invention is an optical system embodying the general conceptof depth-of-field correction taught in a concurrently filed applicationof Edward G. Ramberg, Ser. No. 17,412 and entitled Optical System forOrthogonalizing lmage Field of Projection Lens. ln Ramberg the desireddepth-of-field correction is achieved by the application of the generalconcept of placing a prism in the path of the light reflected from asubject. The particular application, employed by Ramberg, of suchgeneral concept is that of a prism placement adjacent the principalplane of the projecting lens so as to affect substantially only thesubject-reflected light. For monochromatic illumination of thereflecting subject a simple, relatively inexpensive prism located nearthe projecting lens functions satisfactorily. For polychromatic, such aswhite, light subject illumination, however, Ramberg teaches that a prismlocated near the projecting lens needs to be one capable of correctingcolor errors. Such a prism necessarily is relatively complex and, hence,expensive.

The off-axis optical projection system embodying the present inventionand following the broad teaching of Ramberg comprises means forangularly directing a beam of unmodulated, collimated white light onto alight-reflective surface on which is recorded a subject, an image ofwhich is produced on a viewing screen by light-projecting means havingits principal plane substantially parallel to the screen and whichreceives subject-modulated light angularly reflected from thesubject-bearing surface. The projection system also includes a prismlocated adjacent the subject-bearing surface so that it'is in the pathsof both the modulated, collimated light beam and the subject-modulated,reflected light beam, the prism having such configuration, refractiveindex and orientation relative to the real subject-bearing surface as torender the apparent subject-bearing surface parallel, and the lightreflected therefrom orthogonal, to the principal plane of thelight-projecting means and to the viewing screen; and for so reducingany color dispersion of the reflected light as to obtainsatisfactoryresolution of the image of theisubject projected onto thescreen. i

For illustrative purposed the invention, while not necessarily limitedthereto, is embodied in a schlieren type of projection system in whichthe collimated white light is directed by means including a collimatinglens to a reflecting surface on which the subject is recorded asdeformations of the surface. The subject-modulated light'reflected fromthe deformable'surface. is. directed by a projection lens toward theviewing screen and the depth-oflfield correcting orthogonalizing prismis mounted sufficiently close to the subject-bearing reflecting surfacethat whatever color dispersion may he produced by the prism is so smallthat the resolution of the projected image is not adversely affected toa significant degree.

For a more specific disclosure of the invention and its mode ofoperation reference may be had to the following detailed description ofan illustrative embodiment thereof which is given in conjunction withthe accompanying drawings, of which:

FIG. I is a diagrammatic representation of one type of image projectionsystem in which the apparatus of the invention may be used;

FIG. 2 is a fragmentary view, to a grossly enlarged scale, of anelectrode of the system of FIG. 1 on which the subject whose image is tobe projected may be recorded;

FIG. 3 is a diagrammatic representation of the operation of the opticalsystem embodying the invention; and

FIG. 4 is a diagrammatic representation, to a grossly enlarged scale, ofthe operation of the optical system of the invention for typicalcomponent color rays of the white light used in the system, showing theinsignificantly small amount of color dispersion that theorthogonalizing prism tends to produce.

FIG. I shows a general type of image formation and projection system inwhich the invention may be used. In this case the subject, representingthe image to be projected, is formed on a light-reflecting targetelectrode 11 of a cathode ray tube 12 by a video signalmodulatedelectron beam produced by a gun 13 and deflected over the targetelectrode by means including a deflection yoke 14 energized in aconventional manner to scan a raster at the electrode 11. White lightfrom a source 15 is directed by means including a collimating lens 16 atan angle to the surface of the target electrode 11 from which it isreflected in imagerepresentative form and directed by means including aprojecting lens 17 to a viewing screen 18. The particularimage-projecting system shown to illustrate the invention is of theschlieren type which has a stop 19 located substantially at the focalpoint of the projecting lens 17. A depth-of-field correctingorthogonalizing prism 21 is mounted adjacent the target electrode 11 infront of the faceplate of the cathode ray tube 12 so that it is in thepaths of both the collimated light and the subject-modulated lightreflected from the target electrode.

The target electrode 11 of the cathode ray tube 12 is effectively a partofa light valve of the type disclosed in a copending application of JohnA. van Raalte and Victor Christiano. Ser. No. 861,592, filed Sept. 29,1969 and entitled Intelligence-Handling Device Having Means for LimitingInduced Electrostatic Potential. Essentially, as shown in FIG. 2, thetarget electrode 11 comprises an insulating substrate 22, such as glass(which conveniently may be the faceplate of the cathode ray tube) on oneplane surface of which there is a plurality of supporting conductors 23which are electrically connected together (not shown). The spacingbetween adjacent conductors corresponds to the dimensions of anelemental area of the subject to be effectively recorded on theelectrode. The conductors 23 support, in spaced relation to thesubstrate 22, a lightreflective, electrostatically deformable, normallyflat metal film 24 which may be made of alloys of metals such as nickel,copper or aluminum, for example. The

film 24 is sufficiently thin to be pervious to an electron beam 25 sothat pattern of electrical charges 26 may be produced on the insulatingsubstrate 22, the particular pattern being determined by the intensityof the video signal modulation of the electron beam. The electrostaticpotential so produced' between the film 24 and the substrate 22 effectsa local deformation of an elemental area 27 of the film 24. An elementalarea 28 of the film 24 behind which no electrical charges are producedon the insulating substrate 22 remains flat and undeformed.

Again referring to FIG. 1, any light which is reflected from anundeformed elemental area of the metal film 24 of the target electrode11 of the cathode ray tube 12 is intercepted by the schlieren stop 19and, hence, does not reach the screen 18, thereby producing a dark spotat that part of the projected image corresponding in location to hat ofthe undeformed part of the target electrode film. At least some of thelight which is reflected from a deformed area of the target electrodefilm, however, is not intercepted by the stop 19 and, hence, does reachthe screen 18 to produce light of an intensity depending upon the amountof film deformation at the part of the projected image which correspondsto the location of the deformed area of the film in the recordedsubject. The more intense the electron beam is at a given elemental areaof the film the greater will be the film deformation and the greateramount of the light from the source 15 that will be reflected to thescreen 18. In this way a reproduction of the subject recorded at thetarget electrode 11 of the cathode ray tube 12 is projected onto theviewing screen 18. As previously stated, however, such a reproductionwill be subject to such deficiencies as unsatisfactorily low brightness,keystone distortion and the like unless steps, such as the use of theprism 21 embodied in this invention, are taken to compensate for thenecessarily non-orthogonality of the reflected light rays relative tothe principal plane of the projecting lens 17.

The FIG. 3 diagrammatic representation of the optical axes of the systemembodying the invention illustrates the manner in which the prism 21functions to effect the desired depth-of-field correction. In the figurethe lenses l6 and 17 of FIG. 1 are represented as having respectiveprincipal planes 16a and 17a and optical axes 29 and 30. The opticalaxis 29 of the collimating lens 16 is that along which unmodulated whitelight derived from the source 15 enters the front surface 31 of theprism 21 and from which, after refraction, it emerges from the rearprism surface 32 along the axial line 29a. The subject-modulated lightreflected from the target film 24 follows the axial line 30a and entersthe prism 21 at its rear surface 32 to emerge from its front surface 31,after refraction, along the optical axis 30 of the projecting lens 17.The subject-modulated light, thus, is directed substantiallyorthogonally to the principal plane 17a of the projecting lens and tothe screen 18 as if it had been reflected from an apparent target filmsurface 240 along a rearward extension 30b of the optical axis of theprojecting lens. In one successfully operated embodiment of theinvention the prism 21 was trapezoidal in shape, having an apex angle Vof about and the angle A between the optical axes 29 and 30 wasapproximately 30 In the depth-of-field correction system of theinvention, however, the subject-modulated white light directed along theoptical axis 30 of the projecting lens 17 is subject to a slight colordispersion produced by the orthogonalizing prism 21.

FIG. 4 illustrates such color dispersion and the manner in which it isso reduced, by the location of the prism 21 adjacent the reflectingtarget film 24, as to have substantially no significant adverse effectupon the resolution of the image projected onto the viewing screen 18.In the following description the red and blue components of typical raysof the collimated white light directed toward the prism and the filmwill be considered as representing opposite ends of the visible whitelight spectrum. Red and blue ray components 33 and 34, respectively,which are substantially parallel as they impinge upon the front surface31 of the prism 21, emerge from the rear prism surface 32 as red andblue components 33a and 34a and converge at the same point 35 on thefilm 24 because the refraction by the prism is less for the redcomponent 33 than it is for the blue component 34. The reflected red andblue components 33b and 34b emerge from the front surface 31 of theprism 21 as diverging red and blue ray components 33c and 34c,respectively, because of the color dispersion of the prism for such raysof different wavelengths.

By extending the paths of the emerging red and blue ray components 33cand 340 backwards toward the reflecting film 24 as imaginary raycomponents 33d and 34d it is seen that they apparently converge at apoint 36 which is within and adjacent the rear surface 32 of the prism21. Two other red and blue ray components 37 and 38 which impinge uponthe front surface 31 of the prism 21 emerge from the rear surface 32 asray components 37a and 38a and converge at a point 39 on the film 24from which they are reflected as ray components 37b and 38b and emergefrom the front prism surface 31 as diverging red and blue ray components37c and 380 respectively. A backward extension toward the film 24 ofthese diverging ray components as imaginary ray components 37d and 38dindicates their apparent convergence at a point 41 outside the prism 21and adjacent the rear surface 32. The two points 36 and 41 effectivelydefine the color-divergence" plane 42 of the prism from which raycomponents of all colors emerging from the front surface 31 of the prism21 appear to diverge. The amount of such divergence depends upon thecolor (i.e., wavelength) of the ray component, the described redand'blue ray components representing the maximum divergence to beexperienced in practice. It is evident, from a consideration of FIG. 4,that the color-divergence plane 42 and, hence, the prism 21 should lieas closely as possible to the film 24 in order to reduce to aninsignificant degree any color defects in the projected image arisingfrom the color dispersion produced by the prism 21.

Although the main purpose in locating the prism 21 close to thesubject-bearing surface 24 is to effect the desired depth-of-fieldcorrection by orthogonalizing the subject-modulated light relative tothe projecting lens 17 with an insignificant amount of color dispersion, it was found that such a prism location also imtained by orientingthe prism 21 so that its rear surface 32 is substantially parallel tothe subject-bearing surface 24. Such an orientation may, however, resultin some loss of contrast in the projected image, caused by undesiredreflections, but it may be reduced by placing the prism 21 so that itsrear surface 32 is at a small angle (e.g., 2 or 3 relative to thesubject-bearing surface 24. The placement of the prism 21 close to thesubject-bearing surface 24 in accordance with the invention enables theuse for the prism of simple, easily obtained and relatively inexpensivematerials such as lucite, ordinary crown glass and the like having anindex of refraction of approximately 1.5, even though such materialshave relatively high color dispersion constants. The present inventionis applicable to any off-axis optical projection system but when it isused with a cathode ray tube type of light valve, such as that shown inFIGS. 1 and 2, the prism 21 may advantageously be cemented to thefaceplate of the tube, particularly when the faceplate serves as thesubstrate of the light valve. In this way there may be achieved asufficiently close placement of the color-divergence plane 42 (FIG. 4)of the prism relative to the subjectbearing film 24 to still furtherreduce any color dispersion to an absolute minimum.

What is claimed is:

1. An optical system for projecting an image of a light-reflectivesubject having an effective principal plane onto a viewing screen,comprising:

means for directing a beam of unmodulated, collimated white light ontosaid subject, said directing means having an optical axisnonorthogonally disposed relative to the effective principal plane ofsaid subject:

light-projecting means, including a lens having its principal planesubstantially parallel to the plane of said screen and angularlydisposed relative to the effective principal plane of said subject, forreceiving subject-modulated light reflected from said subject anddirecting it toward said screen; and

means for substantially achromatically orthogonalizing saidsubject-modulated light relative to the principal plane of saidlight-projecting lens, said orthogonalizing means comprising a prismlocated adjacent said subject in the paths of both said beam ofunmodulated, collimated white light and said subject-modulated light,

said prism having a rear planar surface closely adjacent to said subjectand oriented to be angularly disposed relative to the principal plane ofsaid light-projecting lens, and a front planar surface angularlydisposed relative to the principal plane of said light-projecting lensas well as angularly disposed relative to the plane of said rear prismsurface, the planes of said front and rear prism surfaces converging ata location on the opposite side of the optical axis of saidlight-projecting lens from the location of the optical axis of saidunmodulated light directing means.

2. An optical system as defined in claim 1,

said prism location placing the effective color-divergence plane fordifferently colored light ray components emerging from the front prismsurface closely adjacent said subject and remote from said principalplane of said light-projecting means.

3. An optical system as defined in claim 2,

said subject being recorded on a deformable film;

said light-projecting means being of the schlieren type having a stoplocated at the focal point of said light-projecting lens.

4. An optical system as defined in claim 3,

said deformable film being a thin metallic sheet supported in spacedrelation to an insulator substrate and constituting an element of alight valve.

5. An optical system as defined in claim 4,

said light valve comprising a cathode ray tube having an envelopeincluding a transparent faceplate enclosing one end of said tube; and

an electron gun mounted within said envelope at its being a singleelement device formed of a material having a relatively high colordispersion constant.

9. An optical system as defined in claim 8,

said single element device having a trapezoidal crosssection, and saidhigh color dispersion constant material having an index of refraction ofthe order of 1.5.

i i k

1. An optical system for projecting an image of a lightreflectivesubject having an effective principal plane onto a viewing screen,comprising: means for directing a beam of unmodulated, collimated whitelight onto said subject, said directing means having an optical axisnonorthogonally disposed relative to the effective principal plane ofsaid subject: light-projecting means, including a lens having itsprincipal plane substantially parallel to the plane of said screen andangularly disposed relative to the effective principal plane of saidsubject, for receiving subject-modulated light reflected from saidsubject and directing it toward said screen; and means for substantiallyachromatically orthogonalizing said subject-modulated light relative tothe principal plane of said light-projecting lens, said orthogonalizingmeans comprising a prism located adjacent said subject in the paths ofboth said beam of unmodulated, collimated white light and saidsubjectmodulated light, said prism having a rear planar surface closelyadjacent to said subject and oriented to be angularly disposed relativeto the principal plane of said light-projecting lens, and a front planarsurface angularly disposed relative to the principal plane of saidlight-projecting lens as well as angularly disposed relative to theplane of said rear prism surface, the planes of said front and rearprism surfaces converging at a location on the opposite side of theoptical axis of said light-projecting lens from the location of theoptical axis of said unmodulated light directing means.
 2. An opticalsystem as defined in claim 1, said prism location placing the effectivecolor-divergence plane for differently colored light ray componentsemerging from the front prism surface closely adjacent said subject andremote from said principal plane of said light-projecting means.
 3. Anoptical system as defined in claim 2, said subject being recorded on adeformable film; and said light-projecting means being of the schlierentype having a stop located at the focal point of said light-projectinglens.
 4. An optical system as defined in claim 3, said deformable filmbeing a thin metallic sheet supported in spaced relation to an insulatorsubstrate and constituting an element of a light valve.
 5. An opticalsystem as defined in claim 4, said light valve comprising a cathode raytube having an envelope including a transparent faceplate enclosing oneend of said tube; and an electron gun mounted within said envelope atits other end, said substrate-supported deformable metallic sheet beingmounted adjacent said faceplate and constituting a target electrode forsaid electron gun.
 6. An optical system as defined in claim 5, saidprism being cemented to said cathode ray tube faceplate.
 7. An opticalsystem as defined in claim 6, said cathode ray tube faceplateconstituting said insulator substrate of said target electrode.
 8. Anoptical system as defined in claim 2, said prism being a single elementdevice formed of a material having a relatively high color dispersionconstant.
 9. An optical system as defined in claim 8, said singleelement device having a trapezoidal cross-section, and said high colordispersion constant material having an index of refraction of the orderof 1.5.